Mischief can answer better than I, but at one emergency vet's
office we used, a cat was kept on the premises for just that purpose.
Poor thing gave blood to all (cat) comers.  I've never seen one
elsewhere.

 Jeanne

Our vet has a donor kitty on site.

Jo

Vets usually keep a cat around, or else they will bring in their
own pets as donors when the need arises. Just a warning, though,
vets are only required to test donor cats 1x per year (and I'm sure
this varies depending on where you live) for FELV or FIV. The donor
cat for Shadow's transfusion was a new cat for the animal hospital
where he was treated, and they supposedly tested him when he came
in, but for some reason they re-tested him after Shadow's
transfusion and found he was FELV+. That's how Shadow became
positive. I know in an emergency situation there isn't always time
to retest a donor, but before choosing an emergency animal clinic
or a specialist, I intend to find out their testing practices way
before/if it becomes necessary.

Not sure about cats but I saw a woman come in to the vet's once with her
dog to give blood.  Said she does so regularly for just to be helpful / a
good samaratin.  Never heard of this since then and no idea if cats do.
It's an interesting thought though - if you have a good healthy indoor
kitty - to offer even if it's only once a year would probably make a
difference in someone's life.
Michelle